The present invention relates to a method for measuring the flow of fluids and more particularly to a method for measuring the velocity of fluid flow or visualizing the distribution (behavior) of a fluid or an intermingled state of two or more different fluids in an engine combustion chamber, a silencer, heat exchanger or the like.
For measurement of the velocity of fluid flow or visualization of the pattern of fluid flow within a confined space such as a pipe for observation of the state of flow, there are known laser Doppler velocimetry (LDV) (using a laser Doppler flowmeter) and particle imaging velocimetry (PIV) (photographing and image processing).
LDV is a method of measuring the flow of a fluid which comprises loading the fluid with tracer particles, projecting two laser beams against the fluid so as to form an interference figure known as "fringe" at the converging point of the beams and observing the scattered light produced as the tracer particles pass through that fringe.
In this method, however, windows of high parallelism (windows whose inner and outer planes are exactly parallel to each other) must be provided for admission of two incident laser beams and, moreover, the inevitable dead angle results in a locality which cannot be observed in the fluid body. Moreover, beyond all else, there is the problem that only the point of convergence of the two laser beams can be observed at a time (point observation).
PIV is a measuring method which comprises loading the fluid with tracer particles, irradiating the fluid using a continuous emission laser or a pulse laser and measuring the velocity of flow or visualizing the distribution of the fluid by photographing and image processing.
Unlike the method comprising the use of converging laser beams, this method does not require windows of high parallelism nor does it have the problem of a non-observable dead angle and permits a broad range of observation in one operation.
However, the scattered light mentioned above includes not only the light originating from the tracer particles, but also the light scattered by interferring objects such as the laser beam incidence window, observation window, pipe wall and suspended dust particles afloat within the pipe.
In actual observation, all of these scattered lights are observed together and the resulting high noise level (low S/N ratio) discourages attempts to measure fluid flows with high accuracy. Particularly when the scattered light from said interfering objects is high in intensity, it is even impossible to track the very tracer particles.
In observing a mixture of two or more fluids for determining the behaviors of the respective fluids or an intermingled state of the fluids, it is impossible to differentially assign the scattered radiations from tracer particles to the respective fluids. Thus, any method observing Mie scattering is not capable of measuring the flow of fluids of this sort. There may be contemplated to vary the size of tracer particles but since there is no perfectly homogeneous laser beam, it is unscrupulous to estimate the size of tracer particles from the intensity of scattered light alone. Moreover, if the size of particles supplied to the fluid is varied, the particle entrainment pattern is also varied. Therefore, the method would not be rewarded with a commensurate benefit.